#!/usr/bin/perl -w
# mycode.pl

use 'modules_src/MyPackage';  # how can do this correctly?

my $start = new Sth::MyPackage;
# etc...

##</code><code>##

~/MyPerl
    |
    |_modules_src
       |_MyPackage.pm
    |
    |_mycode.pl


##</code><code>##

package Sth:MyPackage;

use 5.005;
use strict;
use vars qw( @ISA $VERSION );

# etc2...

1;